Although Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) expression is prototypic of transcripts regulated by their half-lives, how this occurs is not yet well understood. In 1986, Shaw and Kamen reported that an AU rich element (ARE) in the 3' untranslated region of GM-CSF mRNA confers a short half-life on otherwise stable transcripts. How T lymphocytes, which make GM-CSF, use the ARE to accomplish this destabilization remains unclear. We are addressing this question by developing a cell-free system to assay GM-CSF mRNA half-life. The establishment of a simplified, in vitro method for measuring GM-CSF mRNA stability will help us answer two questions: which nuclease(s) contribute to the decay of this mRNA; and how factors known to regulate the half-life of GM-CSF mRNA in cells, for example Tissue Plasminogen Activator (TPA), which has been shown to increase the half-life of GM-CSF mRNA, carry out this regulatory activity. Our laboratory is currently reviewing GM-CSF for reconstitution of bone marrow in patients who have received immunoablative therapy. This product is produced by molecular biological means, either in bacteria or cultured mammaliam cells. Because the product is derived from an inherently unstable mRNA whose stability is acutely regulated by many factors, understanding the nature of and the factors which control GM-CSF mRNA stability is critical for the review of this product.